The replacement of degenerated natural joints with man-made prosthetic replacements is well known, including the replacement of the wrist joint. Prior to the introduction of prosthetic joint replacement, patients with wrist diseases such as radio-carpal arthritis were often treated with a fusion procedure. Fusion typically involved an implantable joint replacement that prevented articulation of the wrist. Fusion however, proved to be less than satisfactory since it left the patient with no movement in the wrist.
Existing prosthetic wrist implants have a number of drawbacks. Because of the size needed to achieve the necessary strength, wrist implants have typically required excessive amounts of bone to be resected. For example, in these implants relatively large and lengthy implant stems were inserted into the radius and carpal bones. This significantly weakened the bones making them more susceptible to post-operative fracture. Furthermore, once fracture occurs, because of the significant loss of bone due to resection, there may not be enough bone left to permit a satisfactory fusion procedure. This may leave a patient without any avenue of treatment, not even fusion.
Another drawback with prior prosthetic wrist implants is that they do not provide the patient with acceptable functionality. Part of the problem is that these implants have not adequately matched the geometry of the natural wrist. As a result, flexion and extension of the hand return to a natural position, and range of motion have all been less than optimum, and the patient is left with less functionality of the implanted wrists than with a natural wrist.
Another difficulty with prior wrist implants involves the means of attaching the implant to the radius and carpal bones. If the attachment is not secure enough, or the implant itself is not strong enough, the stability of the attachment may not be adequate under normal use of the wrist. Because of this, larger implants and implant stems have been used to attach the implant components into the bone. However, as discussed above, this results in more resection and weakening of the bone. As a result of the above issues, existing prosthetic wrist implants have not always achieved adequate levels of patient satisfaction and have sometimes resulted in unacceptable complication rates.
Another problem with wrist implant procedures is the unavailability of special instruments for performing the implant operations. Tools currently available make excavation of a hole, which is cut into the bone to position the implant, difficult and imprecise. As a result, malrotation of the tool can occur which results in misalignment of the implant. Further, specialty guides are unavailable to assist in alignment of a cutting blade during resection of the bone into which the implant is to be fitted. The absence of a guide during the actual resection requires the surgeon performing the resection to “eyeball” the area to be resected. This may result in excess bone material being removed during the resection.
A final problem with prior art wrist implants is the manner in which the implant is pieced together and functions, and the potential to limit the life of the implant due to particle contamination. U.S. Pat. Nos. 5,702,470 and 6,059,832 each describe a prosthetic wrist implant comprising a prosthetic wrist implant disposed between a patient's radius and carpal complex bones. The implant includes a radial implant component, a carpal bone implant component, and an articulating bearing member that is fastened to the carpal bone implant and slidingly engages the radial implant. While these prior art devices may work for their intended purposes, two primary areas could be improved. First, the articulating bearing member of these prior art patents is designed to connect laterally (sideways from the dorsal plane) onto the carpal bone implant component through the use of slots on the bearing member that connect with tabs on the carpal implant. This joining action requires the surgeon to use a large, cumbersome tool to grasp both the bearing member and the carpal implant and slide them together laterally for a tight fit. Second, once the bearing member and carpal implant are joined, the metal surface of the carpal implant is exposed underneath the bearing member. At extreme flexions of the wrist, the metal radial implant can rub against the metal exposed portion of the carpal implant, causing the shedding of metallic particulate matter. This metal particulate matter can contaminate surrounding tissue and lead to cell degeneration and death. Once surrounding tissue is damaged, the implant is loosened and eventually this loosening can lead to implant failure.
Thus, it would be desirable to provide improved prosthetic wrist implants, and methods that overcome some or all of the above-discussed problems. In particular it would be desirable to provide wrist implants with a geometry which matches that of a natural wrist and which affords the patient a natural range of motion, natural flexion and natural extension of the hand. Furthermore, it would be desirable to provide prosthetic wrist implants that are small enough to minimize the bone resection required. It further would be desirable to provide wrist implants that function in a manner so as to minimize effort required to join functioning components of the implants and to also minimize the chances of metal-on-metal contact of implant components and subsequent implant failure. It would also be desirable to provide an improved methods for attaching prosthetic wrist implants which provide a stable and strong attachment to the bone without requiring excessive loss of bone through resection or drilling.